dt-bindings: usb: mtu3: add properties about USB Role Switch

Now the USB Role Switch is supported, so add properties about it,
and modify some description related.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1567070558-29417-5-git-send-email-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>